It has been known for a long time to provide support tubes made of plastic in filter elements, in particular in filter elements for motor vehicles. Support tubes of this kind are also referred to as center tubes and are known, for example, from DE 10 2011 018 366 A1. The support tubes are used to retain or support a filter medium, and the fluid to be filtered or the filtered fluid, in particular air, at least partially flows through them. For this purpose, the support tubes have openings which are separated from one another by longitudinal webs.
With larger designs of support tubes, such as those used in filter elements for trucks for example, it has proved for stability reasons to be advantageous to design the support tube with more than four longitudinal webs. A support tube with a multiplicity of longitudinal webs and openings has become known from EP 1 481 715 A1. The known support tube consists of a plurality of support tube elements which are designed in a basket shape and stacked inside one another. However, to provide such a multiplicity of support tube elements is relatively laborious and therefore expensive to produce and assemble. At the same time, with regard to the production costs, it must be borne in mind that the support tubes in question are mass-produced items, with which even minor improvements in the production of a single support tube have a significant effect on the overall production costs.
It has therefore been shown to be advantageous to design the support tubes in one piece. Such a one-piece support tube is disclosed in DE 10 2005 040 623 A1. The longitudinal webs of this known support tube are round in cross section. However, the support tube can be manufactured even more easily and therefore more cost effectively when the longitudinal webs have a polygonal cross section.
Support tubes of this kind are already manufactured by the applicant. FIG. 2—which will be explained later—shows such a known support tube in cross section. The support tube is produced in a plastic injection molding process and, because of the mold tool used to produce the support tube and for easier removal of the support tube from the mold, it has longitudinal webs with different shape and different cross section. Filter elements with such support tubes are used in filter systems, for example in air filter systems for trucks or construction vehicles, and can usually be fitted in the housing of the filter system with a freely selectable or random rotational orientation about their longitudinal axis. The filter systems frequently have a fluid sensor for determining the filtered fluid flow. Depending on the particular rotational orientation of the filter element and therefore the rotational orientation of the support tube in the housing of the filter system, with the filter elements having the known support tubes, deflections or reversals of the fluid flow in the filter element which are of different strengths can occur depending on the incoming flow to the longitudinal webs. As a result, the fluid flow at the measuring point of the fluid sensor is affected in an unpredictable way.